A study, published in journal Neurology, found that women who give birth to five or more children may be more likely to develop Alzheimer's disease than women who have fewer births.

"Estrogen levels double by the eighth week of pregnancy before climbing to up to 40 times the normal peak level," said co-author Ki Woong Kim frrom the Seoul National University. "If these results are confirmed in other populations, it is possible that these findings could lead to the development of hormone-based preventive strategies for Alzheimer's disease based on the hormonal changes in the first trimester of pregnancy," Kim added.

For the study, the researchers combined the data from two independent population-based studies with a total of 3,549 women. Women -- who were an average age of about 71 at the start of the study -- provided information on their reproductive history. They took the diagnostic examination after an average of 46 years from their first childbirth. During that time, the participants took tests of their memory and thinking skills to see whether they had developed Alzheimer's disease or its precursor, mild cognitive impairment.

The study found that a total of 118 women developed Alzheimer's disease and 896 women developed mild cognitive impairment. The researchers also found that women who had given birth to five or more children were 70 per cent more likely to develop Alzheimer's disease than women who gave birth to fewer children. Women who had experienced an incomplete pregnancy were about half as likely to develop Alzheimer's disease as women who had never had an incomplete pregnancy. Of the 2,375 women who had an incomplete pregnancy, 47 developed Alzheimer's disease, compared to 71 of the 1,174 women who never had an incomplete pregnancy, they added.